In recent years, there has been a great deal of interest in the transmission of video signals via optical fiber. This mode of signal transmission offers a number of advantages over transmitting signals over conventional 75 ohm coaxial cable as video signal distribution is now commonly accomplished in CATV systems. Optical fibers intrinsically have more information-carrying capacity than do coaxial cables. In addition, there is less signal attenuation in optical fibers than in coaxial cable adapted for carrying radio frequency signals. Consequently, optical fibers can span longer distances between signal regenerators than is possible with coaxial cable. In addition, the dielectric nature of optical fiber eliminates any problems with electrical shorting. Finally, optical fiber is immune to ambient electromagnetic interference (EMI) and generates no EMI of its own.
Amplitude modulation of an optical signal with a wideband radio frequency signal requires a light modulating device, such as a laser, which has linear characteristics of a wide dynamic range of operation.
Until recently it has been difficult to fabricate lasers in which the relationship between input current and optical output was linear over more than an extremely limited range. Because of this difficulty in obtaining lasers which were sufficiently linear to support analog amplitude modulation, digital modulation was, until recently, the primary means for transmitting information by optical signals. Digital modulation does not require a laser with a large dynamic range as do analog means for transmitting information (e.g. amplitude modulation or frequency modulation of a carrier frequency modulating the laser output). Digital modulation of the laser offers high signal quality and is also compatible with telephone trunk and feeder networks. However, because of the wideband nature of video signals digitization of these signals consumes extremely large amounts of channel capacity. A typical video signal occupies 6 Mhz of bandwidth. Transmission of this information digitally required a digital data transmission rate of approximately 45 megabits per second. High definition video (HDTV) may require a digital data transmission rate of up to 145 megabits per second. Moreover, encoders and decoders for converting analog video signals to digital form and for reconverting these digital signals to analog form for viewing on a conventional television set are quite expensive. Consequently, analog transmission of video signals is potentially much more economical than digital transmission of such signals.
Recent advances in laser technology have made analog modulation of optical signals feasible. Currently available Fabry-Perot (FP) and Distributed Feedback (DFB) lasers have sufficiently linear characteristics to allow them to be used as analog modulators of optical signals.
One such means of analog transmission is to use the baseband television signal to frequency modulate a radio frequency carrier. This modulated radio frequency carrier is in turn used to modulate an optical signal. Such frequency modulation is less susceptible to noise than is amplitude modulation but it requires more bandwidth for each television channel transmitted than are required by amplitude modulation methods. Thus, the number of television channels which can be carried by each optical transmission (e.g., each optical fiber) in an FM-based system may be somewhat limited. Moreover, since the standard NTSC format for video calls for amplitude modulation of the video carrier, means for converting FM signals to an NTSC amplitude modulated format are required either at the television receiver or at the point at which the fiber transmission trunk is connected to a coaxial cable distribution network. The need for such FM to NTSC AM conversion increases the cost of the system.
In view of the above, a system in which the video baseband signal amplitude modulates a radio frequency carrier signal which in turn amplitude-modulates an optical signal is preferable to other systems from the standpoint of cost and simplicity. However, several phenomena limit the number of radio frequency channels which can be carried by present day optical links where the intensity of light signals is amplitude modulated. A first of these phenomena is a limitation of the amount of radio frequency energy which may be supplied as a modulating signal to a laser or other light generating device before various types of distortions are generated by the light generating device. This power limitation relates to the sum of the radio frequency power contributions of each radio frequency channel. Thus, if it is desired to transmit 80 radio frequency channels over a single optical link, the power available for each channel is only half of the power which would be available if only 40 channels were transmitted by the optical link. Such a limitation on the power of each radio frequency carrier brings each of these carriers closer to the "white noise" level of the system, thus, adversely affecting the signal to noise ratio of the system. Decreasing the number of channels carried by each optical link in order to improve the signal to noise ratio increases the number of lasers which must be used and the overall complexity and cost of the system. On the other hand, increasing the amount of radio frequency power supplied to the laser beyond certain limits may cause the laser to produce several types of distortion which are discussed below.
When the modulating signal supplied to a laser causes the laser to be driven into a nonlinear portion of its input-signal-to-light-output characteristic, harmonic distortion may be produced. The products of this type of distortion are signals which are integer multiples of the "primary" frequency. The second harmonic of 54 Mhz is, for example, 108 Mhz. Thus, if the bandwidth accommodated by a system is such that there are channels at both 54 Mhz and 108 Mhz, second harmonics of the 54 Mhz channel will interfere with the signals on the 108 Mhz channel.
Intermodulation distortion is also of particular concern in amplitude modulated systems. Such distortion results in distortion products at frequencies which are the sum or difference of two other frequencies. The distortion products are the sum difference of two primary frequencies are called second order distortion products and are particularly troublesome. For example, a video channel at 150 Mhz and another video channel at 204 Mhz may produce a second order distortion product at 54 Mhz (the difference frequency) and at 354 Mhz (the sum frequency). Third order distortion products are produced by the mixing of a primary frequency with a second order distortion product. This produces third order distortion products at the sum and difference between the primary frequency and the frequency of the second order distortion product. Third order distortion products may also be generated by mixing signals at three frequencies or by third harmonic generation.
Clearly, one method of dealing with the above problems is to utilize detectors and amplifiers which are highly linear and which are thus relatively insusceptible to harmonic and intermodulation distortion. It is especially important that the production of second order distortion products be minimized. "Optical Receivers" are combinations of such detectors, and amplifiers which serve to convert amplitude modulated light to conventional broadband RF output signals comprising multi-channel video and/or data carriers. Such optical receivers should be effective over a bandwidth of approximately 50 Mhz to 550 Mhz so as to be compatible with current coaxial cable transmission technology. It is desirable that an optical receiver be effective at frequencies greater than 550 Mhz in order to accommodate additional bandwidth which may be required in future CATV systems.
Detectors for converting amplitude modulation of an optical signal to a radio frequency electrical signal corresponding to the modulation may comprise, for example, a photodiode such as the PIN-55D manufactured by PCO Inc. of Chatsworth, Calif. This type of device produces an output current corresponding to the amplitude of light applied to it.
One type of amplifier which has been used for converting the output current signal from such a photodiode to a voltage signal suitable for transmission on a conventional 75 ohm coaxial CATV cable is known as a high impedance amplifier. A generalized schematic of such a high impedance amplifier is shown in FIG. 1. The capacitor C1 of the circuit is essentially a short circuit to radio frequency signals but blocks any DC current from being transmitted. Optical energy from an optic fiber 1 is coupled through a photodiode 2 which acts as an optical power to electrical current converter. This current flows through R1 and R2 producing a corresponding radio frequency (r.f.) voltage signal at the gate of a field effect transistor ("FET") Q1. The output of the FET Q1 drives a 75 ohm coaxial cable through a capacitor C4. A problem associated with the use of such a high impedance amplifier for amplifying a broadband signal is that, at the frequencies in question, the distributed capacitance of the circuit to ground (C.sub.d) coupled with the relatively high input impedance of the circuit tends to attenuate the high frequency response of the circuit ("high frequency roll- off"). Adding additional circuitry to flatten this response can degrade the performance of the circuit with respect to noise and distortion.
A type of amplifier which tends to avoid the high frequency roll-off problem associated with high impedance amplifiers is known as a transimpedance amplifier. A simplified version of such a transimpedance amplifier is shown in FIG. 2. This illustrative transimpedance amplifier is similar to the high impedance amplifier of FIG. 1 except for the addition of a feedback path comprising a resistor R.sub.f and a capacitor C.sub.f between the drain and gate of the field effect transistor Q1. A characteristic of this circuit is that its input impedance is approximately equal to R.sub.f divided by 1 plus the transconductance (G.sub.m) of the circuit (R.sub.f /1+G.sub.m). Thus, depending on the selection of an appropriate resistance value for R.sub.f, the input impedance of a transimpedance amplifier with a gain of 9 may be in the order of 100 ohms as compared with a similar high impedance amplifier which could have an input impedance of 3.5 kilohms or higher. This relatively low input impedance minimizes the problem of high frequency roll-off in the 50 to 550 Mhz frequency band.
Transimpedance and high frequency amplifiers are both susceptible to second order and other even and odd order distortion problems when they are used for the amplification of a high number of television carrier frequencies. In high impedance amplifiers these distortion products tend to be more severe at the low end of the frequency band. In transimpedance amplifiers the problem of second order distortion products is essentially the same throughout the band of operation.
Receiver performance as measured by carrier-to-noise ratio (CNR) and distortions, composite triple beat (CTP) and composite second order (CSO) is generally degraded by non- ideal optical signals. As the optical input power or modulation index increases, the CNR performance of the receiver generally increases, but the contribution of the receiver to the system distortion increases. Conversely, as the optical input power or modulation index decreases, the contribution of the receiver to system distortion decreases, but the C NR performance of the receiver also decreases.
If variations in optical loss occur, the optoelectronic receiver performance may be affected. If the optical loss is greater than expected, the received optical power is lower than expected. Lower than expected received optical power results in a reduced RF output from the photodetector and optoelectronic receiver. Consequently, the input level to the receiver post-amplifier is lower. This condition increases the significance of the noise contribution of the receiver post-amplifier to the system CNR. The final result may be a degradation in system CNR. If the optical loss is less than expected, the received optical power is higher than expected. This results in an increased RF output level from the photodetector and optoelectronic receiver and generally improves the system CNR. However, with the optoelectronic receiver operating at a higher output level, its contribution to the system distortion is greater. The post-amplifier is also operating at a higher level and may contribute further to the system distortion. Consequently, there may be a degradation in system distortion performance.